
// https://leetcode.cn/problems/surrounded-regions/description/

class Solution
{
    int m, n;
    int dx[4] = { 0, 0, 1, -1 };
    int dy[4] = { 1, -1, 0, 0 };

public:
    void solve(vector<vector<char>>& board)
    {
        m = board.size();
        n = board[0].size();
        for (int i = 0; i < m; ++i)
        {
            if (board[i][0] == 'O')
                dfs(board, i, 0);
            if (board[i][n - 1] == 'O')
                dfs(board, i, n - 1);
        }

        for (int i = 0; i < n; ++i)
        {
            if (board[0][i] == 'O')
                dfs(board, 0, i);
            if (board[m - 1][i] == 'O')
                dfs(board, m - 1, i);
        }

        for (int i = 0; i < m; ++i)
        {
            for (int j = 0; j < n; ++j)
            {
                if (board[i][j] == 'a')
                    board[i][j] = 'O';
                else if (board[i][j] == 'O')
                    board[i][j] = 'X';
            }
        }
    }

    void dfs(vector<vector<char>>& board, int i, int j)
    {
        board[i][j] = 'a';
        for (int k = 0; k < 4; ++k)
        {
            int x = i + dx[k];
            int y = j + dy[k];
            if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O')
                dfs(board, x, y);
        }
    }
};